Plastics are ubiquitous and play important roles in industries as well as in people's daily life. Recycled plastic materials provide an inexpensive source of plastics. Proper recycling of plastic wastes and re-processing them into useful materials or articles can not only protect environments but may also create huge economic values. However, recycled plastics are often difficult to reformulate into useable products, especially products with consistent mechanical properties.
Recycled plastics are typically obtained by curbside collection, which itself presents problems as to quality and consistency. The types of plastic materials that are typically designated for curbside recycling are unpigmented high density polyethylene (HDPE) and polyethylene terephthalate (PET), which together constitute about 80% of the collected recycled plastics. Fortunately, some industries have standardized their plastic package materials. For example, plastic milk bottles are made from unpigmented HDPE, while plastic carbonated beverage bottles are made from PET (one-piece containers) or PET/HDPE (two-piece containers). These containers are easily identified and thus are relatively easy to segregate, thereby facilitating the recycling of these two plastics. This is the reason why these two types of plastic are designated for acceptable curbside recycling designated for resin recovery.
It has been demonstrated that recycled plastics, in particular polyolefins, such as HDPE, could be recycled and reprocessed to form useful materials with high economic value (see e.g., Nosker et al, U.S. Pat. Nos. 5,298,214; 5,789,477; 6,191,228; and 7,011,253, which are all incorporated herein by reference). However, new techniques in processing recycled plastics and turning them into useful materials with wider applications are still being actively pursued.